The present invention improves the partial secondary barriers for self-supporting cargo tanks, of vertical axis, axi-symmetrical body shape, to be used on board marine vessels.
The invention is applicable to cargo tanks with any axi-symmetrical shape, with vertical axis, such as a sphere or a cylinder, with spherical or ellipsoidal heads, or a combination of these geometric shapes. All these cargo tanks are mounted on the structure of the said vessels by means of an annular support, joined at its lowermost edge to the vessel's structure and at its uppermost edge to the cargo tank wall; it being also possible for the support to form part of the cargo tank wall in the area where the two join.
Axi-symmetrical cargo tanks of the types mentioned above are especially suited for storing and/or transporting liquefied gases, either at high pressure, or at a pressure slightly above atmospheric. This is due to the fact that their relatively simple geometric form, and the absence of reinforcements that would interrupt the homogeneity of the cargo tank shell, make possible the precise calculation of the stresses at any point on the cargo tank and for any load condition.
All these factors result in increasing the accuracy of the design to such an extent that, for axi-symmetrical cargo tanks whose design is based on the above mentioned type of precise stress analysis, the Classification Societies and other regulatory bodies, permit their construction on board marine vessels. The only requirements are that a partial secondary barrier be provided to contain whatever small leaks that might occur, over a period of about two weeks, as a result of the appearance of cracks of low magnitude, while a complete secondary barrier is required for the other types of tanks transporting liquefied gases.
The possibility of partially eliminating the aforementioned secondary barrier, with the resulting savings in construction costs, is particularly attractive when axi-symmetrical cargo tanks, of the types already mentioned, are used for the marine transport of liquefied natural gas, ethylene, propane, propylene, butane, butadiene, isobutane, ammonia, chrorine and other products which are carried at pressure slightly above atmospheric. This is especially true in those cases where the product to be transported has a very low boiling point, as it is the case for liquefied natural gas carried at-162.degree. C, for ethylene carried at -104.degree. C, and for those liquefied petroleum gases which are transported at a temperature between -40.degree. C and -50.degree. C. In all these cases the cargo tanks are thermally insulated to avoid an excessive evaporation of the product being transported.
One of the methods most commonly used at present to provide a partial secondary barrier consists of lining the hull of a ship, in the zone below the cargo tank with an insulation material that has been covered with a thin metallic sheet capable of withstanding the low temperature of the cargo.
This system has the disadvantage that its construction must take place on board ship and cannot be carried out simultaneously with other work to be done such as hull construction or the assembly of the cargo tanks and their supports. Another drawback of this system is that it requires the insulation of a large area of the hull, thereby increasing the cost of the ship. It also does not prevent cold vapors from the leaking liquid from coming into contact with structural elements of the ship that are made of a quality of steel that is not designed for low temperatures and, moreover, is not thermally insulated.
As is well known, and due to the irregularities of the external surface of the cargo tank wall, there exist interstices of gaps between said surface and the internal surface of the thermal insulation around the cargo tank and along which possible leaks of liquid cargo can travel. Any leak, that occurs at a point on the cargo tank where no such interstice exists, will exert pressure on the thermal insulation and extend over the outer shell of the cargo tank until it reaches an interstice or gap where it can flow freely.
Leaks produced in the tank, above the area where the annular support connects with cargo tank, flow to the vicinity of this zone where pipes are placed to drain off any such leaks. In the area of the cargo tank below the connecting zone with the support, the leaks flow to the lowermost part of the cargo tank where, again, pipes have been placed to collect them. Moreover, there exists in the thermal insulation, in the zones of the tank where drainage pipes are located, a cavity which acts as a collector for the leaks coming from different areas of the cargo tank surface. The pipes coming from both hemispheres of the cargo tank deposit the liquid on the insulation covering the vessel structure in the area below the cargo tank.
The main objective of the present invention is to provide a secondary barrier, for axi-symmetrical cargo tanks of the types already described, whose construction will not interfere with either the construction of the cargo tank or the vessel, or the installation of the cargo tank in the vessel's hull.
Another objective of the invention is to provide a secondary barrier of simple construction and reduced dimensions, when compared to existing secondary barriers, and whose installations on board the vessel will be easily accomplished; all of which will result in a sizeable cost reduction, both for the secondary barrier and the vessel as a whole.
Still another objective of the invention is to provide a secondary barrier that will prevent any structural element of the vessel from coming into contact with either liquid leaking from the cargo tank or the vapors produced by such leaks.
Finally, an objective of this invention is also to provide a secondary barrier that will permit the leaking liquid to be easily disposed of, either in liquid form or in the form of vapors that can be vented to the atmosphere or be used, for example, in the vessel's boilers.
Accordingly, in the present invention, the partial secondary barrier is made up of a tank, preferably cylindrical in shape, whose section is substantially less than the maximum horizontal section of the cargo tank and which is installed below each cargo tank and structurally joined to the inner shell of the double bottom of the vessel.
This tank is made up of a stiff outer wall, an inner thermal insulation and finally, an impermeable layer capable of withstanding the low temperatures of the liquid being collected.
The manner in which the internal insulation is applied makes it possible for the outer wall of the tank to be constructed of normal quality steel. The inner, impermeable layer may consist, for example, of a thin sheet of aluminium.
This tank, which forms the secondary barrier of the present invention, is capable of collecting the leaks from the cargo tank and at the same time fully containing the vapors from the liquid being collected, so that, as a result, there does not exist any danger that a structural element of the vessel will be exposed to the cold vapors from a cargo tank leak.
The inner shell of the double bottom of the vessel may act as the lower base for the lateral external walls of the tank, inside which will be applied the thermal insulation and finally the impermeable inner layer.
The pipes that collect whatever leaks that may occur, discharge into the interior of the tank which constitutes the secondary barrier.
This tank, which constitutes the secondary barrier, may be constructed without an upper cover. In such a case, the walls of the tank are extended upwards to within a short distance from the outer surface of the insulation covering the cargo tank. In the space between the upper edge of the wall of the said secondary barrier tank and the external surface of the said cargo tank, a flexible skirt is installed which may, for example, be a laminated compound made of polyester film and aluminium foil. This skirt prevents the escape of the vapors from the liquid that are being collected and absorbs the thermal expansions and contractions of the cargo tank. If such a flexible skirt is used, then the leak drainage pipes, that originate from points below the area where the cargo tank wall and the annular support connect, have their discharge openings in the zone within the above mentioned flexible skirt. The leak drainage pipe that originate from points situated above the area where the annular support connects with the cargo tank wall, enter the secondary barrier tank through its lateral walls and discharge inside the tank.
In the event that the tank which constitute the secondary barrier is a completely closed structure, all the leak collecting pipes enter the tank through its lateral walls. Moreover, in this case, the pipe or pipes that originate from points on the thermal insulation below the area where the annular support connects with the cargo tank wall will be fitted with expansion joints capable of absorbing the thermal contractions and expansions of the cargo tank.
The tank which constitutes the secondary barrier will be equipped with the means for discharging of the leaked liquid collected.
These means of discharging could consist for example, of an ejector installed inside the secondary barrier tank and activated by one of the discharge pumps of the cargo tank.
The discharging of the leaked liquid collected in the secondary barrier tank may also be accomplished by injecting a warm gas, such as nitrogen, into the tank thereby causing the collected liquid to evaporate. The vapors produced by this evaporation would then be either vented to the atmosphere, sent to the ship's boilers, or burned in a catalytic burner.
The evaporation of the liquid in the tank could also be achieved by installing one or more coils inside the tank and through which a warm fluid would be circulated. The vapors thereby produced, as in the aforementioned case, would then be vented to the atmosphere or sent to the ship's boilers.
In the bottom of the secondary barrier tank one or more sump wells can be installed. For example, two sump wells can be located symmetrically with respect to the center longitudinal plane of the ship in the aft part of the secondary barrier's base. These sump wells will coincide with and fit into two cavities already present in the inner shell of the double bottom of the vessel.
The sump wells in the secondary barrier tank serve to centralize the liquid being collected, to increase the capacity of the tank, and, finally, to provide a location for the installation of the previously mentioned ejectors and coils (or pipes for injecting warm gases).